The invention relates to a color television signal receiver for demodulating an NTSC-coded chrominance signal available as a sequence of digital values, including an oscillator circuit producing two reference signals of the chrominance subcarrier frequency and being shifted relative to each other, the oscillator circuit comprising an accumulator incorporating a register and a (first) modulo-adder and a waveform converter which forms the reference signals from the accumulator output signal, and a phase comparator circuit controlling the phase of the reference signals and receiving during the occurrence of the color synchronizing signal a signal produced in at least one demodulator multiplying the chrominance signal by at least one of the reference signals.
Such a circuit arrangement is disclosed in the Collection of papers of the 15th annular conference of SMPTE in San Francisco, February 1981, pages 200-209, entitled: Digital Decoding of PAL and NTSC Signals Using Field Delay Comb Filters and Line-Locked Sampling, Author: C.K..P. Clarke. For the demodulation of the color signal it is necessary that the frequency and phase of a reference signal are controlled such that its phase corresponds to the phase of the color synchronizing signal. With NTSC-encoded composite color television signals, the phase position of the color producing part of the chrominance signal frequently changes on the transmission path relative to the color synchronizing signal, as the color synchronizing signal is always located at a constant basic level, namely on the back porch, but the color producing part of the chrominance signal is always superposed on the continuously fluctuating luminance signal. Such a phase shift of the color producing part of the chrominance signal relative to the color synchronizing signal is visible in the displayed picture as a hue change. The prior art circuit arrangement does not provide the possibility to compensate for this phase error.
In NTSC-television receivers the phase position of the color synchronizing signal is often adjusted, for example to offset the phase shift occurring on the transmission path between the color producing part of the chrominance signal and the color synchronizing signal. European Patent Application EP No. 00 67 999 discloses, for example, a circuit arrangement in which the output signal of the comparator circuit is applied to the analog oscillator via a digital-to-analog converter and in which the phase shift can be controlled by a user of a television receiver. By manual setting the characteristic of the phase comparator circuit is influenced and a different desired phase position is determined for the color synchronizing signal.